1. Field of the Invention
This invention generally relates to zinc titanate-containing compositions used to remove reduced sulfur gases such as H2S, COS, and CS2 from gas streams. More specifically, it relates to those zinc titanate-containing compositions that, aside from their chemical reactivity toward reduced sulfur gases, also have she physical attributes of toughness and attrition resistance.
2. Background of the Invention
Reduced sulfur gases are present in many industrial processes. For example, reduced sulfur gases are found in flue gas, coal gas and fuel gas streams. They are also found in industrial product gas streams such as olefin-containing gas streams which are a component of petroleum refining operations. These gases are often removed from such gas streams by vise of various metal oxides that have the ability to capture a reduced sulfur-containing gas component from such gas streams. In order to capture such a gas from certain industrial processes (such as packed-bed, fluidized-bed or moving-bed reactors) the metal oxide, reduced sulfur gas sorbent materials must be used in forms which are mechanically strong and resistant to attrition. Otherwise, problems such as pressure drops through a process reactor unit, particulate matter elutriation and/or clogging of valves or other mechanical components will take place.
Moreover, almost all industrial processes that deal with reduced sulfur gases also are confronted with the problem of desorbing these gases from the metal oxide sorbent material so that said sorbent material can be used over and over again in order to obtain its maximum economic benefit. Other problems associated with the presence of reduced sulfur gases (such as H2S, COS and CS2) in gas streams such as fuel gases, flue gases and waste gases arise from the fact that reduced sulfur gases are corrosive toward ferrous metals. They are especially corrosive toward steel turbine blades. Therefore, the presence of reduced sulfur gases in those hot fuel cases used to power turbines results in their severe corrosion. Oxidation of hot fuel gases also serves to oxidize any reduced sulfur gases contained therein. The resulting sulfur oxide gases (e.g., SO2 and SO3 which are commonly referred to as xe2x80x9cSOXxxe2x80x9d gases) also are highly corrosive toward ferrous metals. Moreover, upon release to the atmosphere, SOx gases form so-called xe2x80x9cacid-rain.xe2x80x9d Therefore, the concentration of reduced sulfur gases contained in those hot fuel gases introduced into power producing equipment such as turbines and fuel cells must be brought to very low concentrations, e.g., a is few parts per million (ppm), before they are combusted in equipment of this kind.
Next, it should be noted that, in the case of sulfur oxide sorbentsxe2x80x94as opposed to the reduced sulfur sorbents that form the subject matter of this patent disclosurexe2x80x94the subject sulfur oxide is normally usually captured in an oxidizing atmosphere such as those extant in the catalyst regenerator of a FCC unit. This is done through use of various metal oxide particles having an affinity for a given sulfur oxide-containing gas. These particles are often entrained in a xe2x80x9cfluidizedxe2x80x9d process. For example, sulfur oxide (e.g., SO2 or SO3) sorption is often carried out through use of fluidized microspheroidal magnesium-containing particles that can withstand the hot (e.g., 1350xc2x0 F.) oxidizing conditions present in the catalyst regenerator units of those fluid catalytic conversion (xe2x80x9cFCCxe2x80x9d) processes used to refine petroleum. Conversely, release of such sorbed sulfur-containing gases usually occurs in the reducing environment of a FCC reactor. The sulfur component of such gases is released as hydrogen sulfide H2S. This released H2S gas is readily captured downstream of the reactor and normally does not create an environmental hazard.
Again, however, the processes of the present patent disclosure are different from such So, sorption processes in that applicant""s compositions are specifically designed to capture chemically reduced forms of sulfur (e.g., those in H2S, COS and CS2) rather than chemically oxidized forms of sulfur (e.g., those in SO2 and S3O). Thus, applicant""s capture of reduced sulfur gases must take place under chemical reduction conditions, rather than under chemical oxidizing conditions.
Many different zincxe2x80x94containing compounds have been used in both fixed bed and fluid bed systems in order to remove one or more species of reduced sulfur gases from various industrial gas streams (e.g., fuel gases, such as those derived from the gasification of coal, flue or waste gases and/or industrial product gases such as those that contaminate olefin-type gases). Such zincxe2x80x94containing compounds have included zinc oxide, zinc titanate and zinc aluminate. Zinc oxide, for example, has been used as a sorbent for selectively removing hydrogen sulfide gas. H2S, from certain industrial gas streams. This metal oxide is normally used by placing it in contact with a hydrogen sulfide-containing gas stream at elevated temperatures. Zinc oxide, in and of itself, has not, however, proven to be a particularly effective hydrogen sulfide sorbent for many industrial applications. For example, its hydrogen sulfide sorption ability is relatively limited, especially at lower temperatures. It also suffers from the drawback of not being easily regenerated. This drawback follows from the relatively high thermodynamic stability of the zinc sulfide product of zinc oxide-hydrogen sulfide reactions. Zinc oxide also lacks the qualities of hardness, toughness and/or attrition resistance that are needed for many industrial applications.
Regeneration of the zinc sulfide product of zinc oxide-hydrogen sulfide reactions requires subsequent oxidation of the sulfur component of the zinc sulfide reaction product. This must be done at relatively high temperatures (e.g., 900xc2x0 F. to 1500xc2x0 F.). Unfortunately, the relatively high temperatures needed to oxidize zinc sulfide back to zinc oxide also tend to degrade the already inherently low mechanical strength and/or toughness of these zinc oxide-based materials. Consequently, zinc oxide sorbents tend to quickly disintegrate hen they are repeatedly used and regenerated.
Therefore, in order for zinc oxide-containing compounds to be effectively used in the harsh environments where they are needed (e.g., in the high temperature/high velocity particle impact environments of fluid, fixed or bubbling bed processes), they must be combined with other tougher and more attrition resistant metal oxide components in order to produce overall zinc oxide/metal oxide compositions having the requisite mechanical strength, hardness, durability, toughness and attrition resistance that they will need to function as reduced sulfur gas sorbents.
Generally speaking, this has been accomplished by mixing certain prescribed proportions of a relatively soft zinc oxide component with certain prescribed portions of another, relatively harder, tougher, metal oxide component in the same particle. The most effective and widely used metal oxide used for this hardening/toughening purpose has been unreacted alumina (Al2O3). Such use of alumina as a catalyst support for zinc oxide sorbents follows from the unusually high degree of hardness this material imparts to such compositionsxe2x80x94as well as from the excellent binding capabilities of many forms of so-called xe2x80x9cgellingxe2x80x9d or xe2x80x9csolxe2x80x9d alumina forms. Examples of such aluminas are the various grades of VISTA CATAPAL and CONDEA DISPERSAL aluminas. Such aluminas have been used in producing relatively harder, tougher and more attrition resistant extrudate, granule, microsphere, powder, particle, pellet, bead, etc. forms of zinc oxide/unreacted alumina compositions.
Many of the above-noted improvements in the sorption, regeneration and physical attributes of zinc oxide-containing compositions are taught in the patent literature. For example, U.S. Pat. No. 4,088,736 (xe2x80x9cthe ""736 patentxe2x80x9d) discloses a reduced sulfur sorbent comprised of homogenous mixtures of zinc oxide, alumina, silica, and Group II-A metal oxide(s). The alumina and silica components of the compositions taught by the ""736 patent serve to impart toughness and attrition resistance to the therein disclosed compositions.
Other patent disclosures teach the use of other zinc-containing compoundsxe2x80x94that is to say, other than zinc oxidexe2x80x94as the xe2x80x9cactive,xe2x80x9d reduced sulfur gas-capturing agent in such compositions. For example, U.S. Pat. No. 4,263,020 (xe2x80x9cthe ""020 patentxe2x80x9d) discloses the reduced sulfur gas-capturing abilities of metal aluminate spinels having the general formula MAl2O4 wherein the M component can be chromium, iron, cobalt, nickel, copper, cadmium or mercury. Zinc aluminate spinel, ZnAl2O4 is a particularly preferred member of this group of compounds. The ""020 patent also notes that the zinc atoms of such a zinc aluminate spinel form simple adsorption bonds with reduced sulfur gases. These adsorption bonds are sufficient to remove a reduced sulfur gas, such as hydrogen sulfide, from a recycle hydrogen gas stream. The ""020 patent also emphasizes that, unlike the chemical mechanism involved in the removal of reduced sulfur gas (e.g., hydrogen sulfide) from a recycle hydrogen gas stream by the use of zinc oxide-based sorbents, here is no chemical reacting in the process of the ""020 patent wherein zinc sulfide is ever formed from these MAl2O4 compounds. Consequently, they can be regenerated by simply purging or sweeping the physically sorbed, reduced sulfur gas from these MAl2O4 compounds with a hot, inert gas such as nitrogen.
Zinc titanate has also been used as a reduced sulfur compound sorbent. Indeed, it has been used in sorbents having no binder component other than the zinc titanate itself. Unfortunately, zinc titanate (like zinc oxide) also suffers to some degree from the drawback of being a relatively xe2x80x9csoftxe2x80x9d material. Consequently, most zinc titanate-containing compositions (like zinc oxide-containing compositions) employ an unreacted alumina component as a binder material in order to create hard, tough, attrition resistant zinc titanate/unreacted alumina compositions.
Still other zinc titanate-containing, reduced sulfur sorbent compositions do not employ unreacted alumina, but rather employ other kinds of toughness-imparting binder materials. For instance, U.S. Pat. No. 5,254,516 (xe2x80x9cthe ""516 patentxe2x80x9d) teaches various zinc titanate-based sorbent materials that further comprise a combination of inorganic and organic materials that are used as a binder for the active zinc titanate ingredient. These inorganic binder materials include clays such as kaolinite and bentonite (which are aluminosilicates), feldspar, sodium silicate, forsterite and calcium sulfate. The more preferred organic binders used to create the tough binder materials disclosed in the ""516 patent are methylcellulose-based compositions such as that commercially available as METHOCEL.
Those skilled in this art also will appreciate that zinc titanate-containing compositions tend to lose more and more of their capacity to absorb reduced sulfur gases as more and more of the binder ingredients (e.g. clay, alumina, organics, etc.) are used in these compositions. It might also be noted in passing here that it was heretofore generally believed that the reason for this loss in reduced sulfur gas capturing ability was simply due to the fact that less zinc titanate is present in those compositions having relatively greater proportions of binder materials.
Applicant has, however, learned that the above noted reduced sulfur-capturing capacity is only partially due to the reduced concentrations of such sulfur sorbent materials. Indeed, applicant has established that the reduction of sulfur sorbing capacities in such materials is largely caused by certain adverse chemical interaction between the zinc titanate and the other components of the sorbent particle (such as their binder and filler ingredients). Applicant has also discovered the most undesirable of these reactions is one that occurs between zinc titanate and unreacted alumina. That is to say that even though certain metal oxides, and especially unreacted alumina, have proven to be especially effective binders for zinc titanate-based compositions because they serve to greatly improve the mechanical strength and attrition resistance of the resulting zinc titanate/unreacted alumina particles, applicant has found that such use of unreacted alumina results in overall reduced sulfur gas sorbent compositions that exhibit progressively lower reduced sulfur gas capturing activityxe2x80x94not only because more and more unreacted aluminum-containing compounds (such as alumina, clay, etc.) are added to these compositions in order to make them harder, tougher and more attrition resistantxe2x80x94but because some of the aluminum component of she unreacted alumina has chemically reacted by some of the zinc component of the zinc titanate.
In other words, applicant has found the reason behind the fact that, despite the improvements in the physical attributes of zinc titanate-containing particles in general, and especially these brought about by use of xe2x80x9coptimalxe2x80x9d proportions of unreacted alumina binders, prior art reduced sulfur sorbent compositions are characterized by the fact that they, all too soon, lose their reduced sulfur-containing gas sorbent ability and/or their physical integrity, as they are repeatedly used, regenerated and reused. Armed with this understanding, applicant has produced reduced sulfur gas sorbents that are characterized by their relatively better reduced sulfur gas sorbent abilities and physical hardness, toughness, durability and/or attrition resistance qualities. It is therefore an object of the present invention to describe certain reduced sulfur sorbent compositions that are simultaneously capable of readily reversibly sorbing, and releasing, relatively large amounts of reduced sulfur gas without losing their sulfur sorbing ability and/or quickly succumbing to the harsh conditions where these particles are employed.
Indeed, applicant""s reduced sulfur gas sorbent compositions even tend to gain in their reduced sulfur gas sorbent abilities as they are repeatedly used over many successive sorption and regeneration cycles. Hence, the reduced sulfur sorbent compositions of this patent disclosure are especially well suited for use in a wide variety of production and/or pollution control process streams (e.g., removing a reduced sulfur gas from a hydrocarbon stream, e.g., removing H2S from an olefin stream, removing reduced sulfur gases from flue gases, etc., removing sulfur from coal gases before they are introduced into a turbine, etc.). Applicant""s compositions are especially useful in those bubbling and fluid bed processes wherein the mechanical stresses imparted to particles of such reduced sulfur sorbents compositions are severe, and, if not in some way guarded against, would result in high elutriation losses from any process employing these compositions under such adverse conditions.
In summary, the :ore desirable properties of the reduced sulfur sorbents taught by this patent disclosure include their improved (1) hardness, toughness and attrition resistance, (2) ability to capture reduced sulfur gases such as H2S) from a variety of gaseous streams, (3) ability to release the sorbed sulfur species, (4) ability to minimize sorbent deactivation over relatively more cycles of sorbing and releasing Various reduced sulfur species, (5) ability to form into special shapes (i.e. microspheroidal particles) that are particularly useful for certain applications, (6) ability to remove reduced sulfur gases from a stream of a commercially valuable gas product (e.g., remove H2S from an olefin stream) and (7) special suitability for capturing reduced sulfur gases from hot fuel gas streams such as those used to power turbines and fuel cells (e.g., removing reduced sulfur gas from a coal gas stream before it is introduced into a turbine).
Applicant""s experimental work has established that the loss of activity of zinc titanate/unreacted alumina compositions with respect to their ability to pick-up reduced sulfur gases is due, in large measure, to formation of a zinc aluminate phase in such compositions during their use in high temperature environments (e.g. those higher than about 500xc2x0 F.). This zinc aluminate phase forms from the zinc component of the zinc titanate active ingredient and from the aluminum component of those unreacted alumina ingredient(s) normally used as binder ingredients for the zinc titanate in most prior art reduced sulfur sorbent compositions. Applicant also has found that this zinc aluminate phase is not normally formed under the sorption cycle in which the chemically reduced sulfur gas is sorbed, but rather is, for the most part, formed during the subsequent relatively higher temperature, regeneration cycle wherein the sorbed, reduced sulfur gases are driven off the composition so that it can be reused over and over again.
Applicant also found that this zinc aluminate forming chemical reaction is virtually irreversible under the catalyst regeneration temperature conditions that exist in most processes wherein such zinc titanate/unreacted alumina compositions are employed. In effect, this loss of activity toward reduced sulfur gases follows from the fact that the newly formed zinc aluminate phase possesses significantly lower reduced sulfur gas capturing ability relative to that of the original zinc titanate ingredient of such compositions. Indeed, applicant""s experimental work indicates that the reduced sulfur gas activity of this zinc aluminate phase is often as much as an order of magnitude less than that of the original zinc titanate ingredient. In other words, this newly formed zinc aluminate phase can be thought of as xe2x80x9cpoisoningxe2x80x9d the sulfur sorbing zinc titanate ingredient of such zinc titanate-containing compositions. Worse yet, this poisoning effect becomes more and more pronounced as these compositions experience repeated sorption/regeneration cycles.
Thus, this invention is particularly concerned with preventing degradation of the active, reduced sulfur gas capturing, zinc titanate phase in those zinc titanate-containing reduced sulfur gas sorbent compositions that also employ aluminum ingredients (e.g., alumina-based ingredients) in order to give such compositions the hardness, toughness and attrition resistant qualities they need to survive in the harsh environments where they are employed. Applicant""s invention also may be considered as teaching a method of producing attrition resistant zinc titanate-containing compositions (e.g., microspheroidal particles) by using a metal oxide-aluminate phase to support (bind, etc.) the active zinc titanate phasexe2x80x94as opposed to using an unreacted alumina phase for this support (binding, etc.) function.
The improved reduced sulfur gas capturing ability and desired physical characteristics of applicant""s compositions are simultaneously achieved by chemically incorporating another metal-containing compound into an aluminum-containing binder componentxe2x80x94and especially into an unreacted alumina binder componentxe2x80x94of an overall zinc titanate/metal oxide-aluminate phase composition. That is to say that a chemical reaction is produced between at least a portion of an aluminum-containing component (such as unreacted alumina) and a metal-oxide containing compound (such as magnesium oxide) so that a resulting metal oxide-aluminate phase (e.g., MgO.Al2O3) will not thereafter chemically react with any zinc oxide driven off the zinc titanate component during regeneration of these sulfur sorbents at those elevated temperatures (e.g., those greater than about 700xc2x0 F.) at which these sulfur sorbents are normally regenerated.
In other words, the metal oxide component of applicant""s metal oxide-aluminate phase ingredient is further characterized by the fact that it is, to some degree, already chemically reacted with an aluminum-containing compound such as unreacted alumina when the sulfur sorbent experiences the high temperatures at which these sulfur sorbents are regenerated. Applicant will emphasize that this binder component of these overall compositions is not merely a mixture of the metal oxide (e.g., MgO) and unreacted alumina (Al2O3), but rather is a compound formed from these chemicals, through applicant""s use of the expression xe2x80x9cmetal oxide-aluminate phasexe2x80x9d. That these chemical reactions have, in fact, occurred can be verified in several ways. For example, the XRD pattern for the resulting metal oxide-aluminate phase will differ from the XRD pattern of the subject metal oxide compound (e.g., MgO) itself, as well as from the XRD pattern of the subject unreacted aluminum-containing compound (e.g., Al2O3) itself. For example, if the metal oxide is magnesium oxide (MgO) and the unreacted aluminum-containing compound is alumina (Al2O3), the XRD pattern of applicant""s resulting metal oxide-aluminate (MgO.Al2O3) phase will differ from that of the metal oxide (MgO) and from that of the unreacted alumina (Al2O3) Thus, applicant""s overall zinc titanate/metal oxide-aluminate phase composition may be thought of as a zinc titanate phase that is combined, mixed, associated, etc. with a metal oxide-aluminate phase in the same particle. Again, the object of applicant""s processes is to prevent chemical reactions between the zinc component of a zinc titanate and the aluminum component of the unreacted alumina (Al2O3) under those high temperature conditions where reduced sulfur gas sorbents are employed. Applicant""s reduced sulfur gas sorbent composition also may contain certain optional Ingredients hereinafter more fully described.
Next, it should be noted that an excess metal oxide phase or an excess alumna phase may be present in applicant""s overall reduced sulfur gas sorbent compositions (that is to say that complete chemical reaction between all of applicant""s aluminum-containing compound (e.g. unreacted alumina) and all of applicant""s metal oxide (e.g. MgO, ZnO, etc.) is not necessary for effective formulation of the overall compositions that constitute the subject matter of this patent disclosure. Complete chemical reaction between these metal oxide and alumina ingredients may, however, in many cases, be preferred.
Examples of the metal oxide-aluminate phases that can be used in applicant""s sulfur sorbent compositions are varied and extensive. Indeed, such metal oxide-aluminate phases can be made with any metal or metal compound (e.g., a metal oxide) that can, to some extent, chemically react with alumina. These metals may include, but are not limited to, Mg2+, Ca2+, Zn2+, Ni2+. Bivalent metals are, however, particularly preferred for this purpose. The oxides of these metals (e.g. MgO, CaO, ZnO, NiO) as well as their nitrates, acetates, etc. forms can be used as starting materials for applicant""s formulations. Those skilled in this art will, however, appreciate that the non-oxide forms of these metal compounds (e.g., their nitrates, acetates, etc.) will be converted to oxide forms, (e.g., MgO, CaO, ZnO, NiO, etc.) when these non-oxide metal compounds are subsequently subjected to certain high temperature processes (e.g., calcining) that may be used in the manufacture of these reduced sulfur gas sorbentsxe2x80x94or which may be encountered (e.g., in high temperature regeneration units) during actual use of these sorbents.
Next, it should be noted that applicant also has found that, if the metal oxide-aluminate compound used in the processes of this patent disclosure is zinc aluminate, ZnAl2O4, the resulting zinc titanate/zinc oxide-aluminate composition is not poisoned by the original presence of the zinc aluminate. Applicant believes that this seemingly anomalous result follows from the fact that since the zinc aluminate phase, is already present in the zinc titanate-zinc aluminate sorbent particle, it prevents the zinc component of the zinc titanate from reacting with the aluminum component of the zinc aluminate. Consequently, following release of the sorbed sulfur species from such compositions, the zinc component of the zinc titanate compound again recombines with the titania to once again form zinc titanate which is again ready for reduced sulfur gas sorbing duty in a subsequent sorption cycle. In effect, this particular metal oxide-alumina chemical reaction produces a phase (a metal oxide-aluminate) that is not reactive toward the zinc titanate active ingredient.
Applicant""s sulfur sorbent compositions will preferably contain, in the same particle, from about 5 to about 30 weight percent zinc titanate, and from about 20to about 95 weight percent of the metal oxide-aluminate phase. The most preferred concentration will depend, in large part, on the particular end use intended. For instance, in those environments that employ a fluid bed process, the particles require maximum attrition resistance. Consequently, a high concentration of the metal oxide-aluminate phase may be required. Conversely, in fixed bed applications, the attrition resistance of the sorbent particles are relatively less importantxe2x80x94while the overall sulfur sorption capacity is relatively more important.
It might also be noted here that, for purposes of this patent disclosure, the term(s) xe2x80x9cparticle(s)xe2x80x9d will be employed to describe any one of, or all of, the extrudate, granule, microsphere, powder, particle, pellet or bead forms of the compositions of this patent disclosure. These particles also may contain minor amounts (e.g., less than twenty weight percent) of other auxiliary ingredients such as other binder materials (i.e., other than the metal oxide-aluminate phase), fillers, fluxes, surfactants and gas evolution agents.
In some cases, the resulting metal oxide-aluminate phase component of applicant""s overall zinc titanate phase/metal oxide-aluminate phase compositions may have some capacity to remove reduced sulfur gases in its own right, while in other cases, the resulting metal oxide-aluminate phase will possess no reduced sulfur gas sorption capabilities whatsoever. In either case, however, the resulting zinc titanate phase/metal oxide-aluminate phase, sulfur sorbent, compositions of this parent disclosure will have improved reduced sulfur sorbent capabilities relative to those prior art compositions comprised of similar concentrations of zinc titanate and unreacted alumina that exist as a mixture of zinc titanate and a mixture of metal oxide (e.g., MgO) and alumina (Al2O3) that have not been chemically reacted.
Indeed, the properties (chemical as well as physical) of applicant""s reduced sulfur gas sorbents compare very favorably to a wide variety of sulfur sorbent compositions prepared by various prior art methods. For example, applicant has established that the sulfur sorbent compositions of this patent disclosure have better reduced sulfur gas activities and attrition characteristics relative to those based upon the use of (1) binderless zinc titanate compositions, (2) compositions comprised of zinc titanate and unreacted alumina mixtures and (3) compositions based upon the combined use of zinc titanate and other commonly used inorganic binder materials such as kaolin and bentonite.
Another extremely important aspect of the present invention is the fact that applicant""s zinc titanate/metal oxide-aluminate phase compositions will actually improve in their activity toward reduced sulfur gas species over repeated cycles of use. This behavior with respect to the effects of regeneration upon applicant""s reduced sulfur capturing compositions stands in stark contrast to the fact that, under otherwise comparable conditions, repeated cycling of all prior art compositions known to applicant (such as those based upon the use of mixtures of zinc titanate and unreacted alumina) leads to their early, accelerating and very significant deactivation with respect to their ability to pick up reduced sulfur gases. The reason why applicant""s compositions actually become better reduced sulfur-capturing agents upon repeated use is not fully understood; however, applicant believes this phenomenon may be due to certain synergistic effects between the zinc titanate and the metal oxide-aluminate phase that take place upon successive sorption/regeneration cycles. Indeed, this improvement in reduced sulfur gas sorbing ability may be taken as some evidence that a chemical reaction has been carried out between applicant""s metal oxide and alumina ingredients. In improved reduced sulfur gas-capturing ability over success use also may be due, at least in part, to increased xe2x80x9cactivationxe2x80x9d of the zinc titanate or metal-oxide aluminate phase with successive sorption/regeneration cycles, One experimental result supporting the latter view was applicant""s repeated observation of an increase in overall surface area of such compositions following multiple sorption/regeneration cycles. Those skilled in this art will appreciate that an increase in the surface area of such a sulfur sorbent particle may serve to increase its reduced sulfur gas sorbent capabilities. Some additional experimental data (e.g., XRD traces for the zinc titanate component of these compositions) also suggests that a decrease in the crystalline sizes of the zinc titanate is taking place. This too, may play some role in the improved sulfur-capturing ability of applicant""s compositions as they are repeatedly used.
Applicant believes that the reason(s) for the relatively faster and more severe chemical deactivation of those prior art zinc titanate sorbents used in conjunction with an unreacted alumina that is used as the zinc titanate""s binder material can be summarized by the following generalized reactions:
In these tables, ZT represents a zinc titanate phase and ZA represents a zinc aluminate phase. Again, a ZT phase material made and employed according to the scheme of Table IA is generally characterized by both a relatively high reactivity toward reduced sulfur gas species and by a relatively low degree of toughness and attrition resistance. Conversely, those prior art ZT+Al2O3, compositions depicted in the chemical reaction scheme of Table IB are generally characterized by their relatively greater toughness and attrition resistance, but relatively lower chemical activity toward reduced sulfur gas speciesxe2x80x94especially over repeated cycles of use (relative to the materials depicted in Table IA). By way of contrast, applicant believes that the chemical reaction mechanism of the compositions of the present patent disclosure (as they undergo chemical reaction with a reduced sulfur-containing gas species such as H2S) can be generally described by the reaction schemes depicted in Table IIA and IIB.
In Table IIA, the metal oxide-aluminate component of such compositions is assumed to be is substantially inactive toward reduced sulfur gas species while in Table IIB, such a metal oxide-aluminate phase is assumed to be chemically active toward such reduced sulfur species.
In these tables, ZT represents a zinc titanate phase, MS represents a metal sulfide phase and MOA represents a metal oxide-aluminate phase. In either case, however, no significant reduction in the ZT phase results following successive sorption/regeneration cycles. Hence, no zinc aluminate is formed. Consequently, the zinc titanate (ZT) phase is not xe2x80x9cpoisoned.xe2x80x9d
Next, it should be noted that the zinc titanate/metal oxide-aluminate phase compositions of this patent disclosure can be made by two general methods. The first general method involves pre-reacting a zinc-containing compound with a titanium-containing compound at a sufficient temperature to effect a transformation of the zinc and titanium containing species into a zinc titanate compound. The various techniques for doing this are well known to this art. Indeed, in each of the two general methods for making applicant""s zinc titanate/metal oxide-aluminate compositions, the zinc titanate ingredient may be obtained from commercial sources.
In any case, applicant""s first production method (A), starts with mixing zinc titanate and one or more metal oxide-aluminate phase compounds in the presence of various inorganic binders and one or more liquid binder solutions A zinc titanate phase/metal oxide-aluminate phase precursor composition resulting from such mixtures is then formed into desired shapes such as extrudates, microspheres, granules, pellets, powders, extrudates or powders. The resulting physical forms of these materials are then subjected to a temperature greater than about 300xc2x0 C. for a time period of greater than about 1 minute to convert the precursor components into a zinc titanate/metal oxide-aluminate composition with the desired physical properties (e.g., toughness, attrition resistance, macroporosity and surface area). More preferably, however, the heating period will be from about 1 to about 2 hours (at temperatures ranging from about 300xc2x0 C. to about 1200xc2x0 C.). Alternatively, where the targeted process operates at sufficiently high temperatures to effect the desired chemical phase transformations, this heat treatment step may occur within the process reactor vessel itself.
In the second manufacturing method (B), a zinc-containing compound, an aluminum-containing compound (and especially an unreacted alumina-containing compound) and the metal oxide-containing compound are mixed with various liquids to form a slurry, paste, etc. The resulting composition is then formed into a desired shape (e.g., extrudates, microspheres, granules, pellets or powders. The resulting physical forms of these compositions are then subjected to a temperature greater than about 300xc2x0 C. for a time period greater than 1 minute to convert the precursor components into the zinc titanate/metal oxide aluminate phase composition with the desired physical properties such as toughness, attrition resistance, macroporosity and surface area. Preferably, this heating period will be from about 1 to about 2 hours at temperatures ranging from about 300xc2x0 C. to about 1300xc2x0 C. Alternatively, where the process in which applicant""s sulfur sorbents is to b used operates at sufficiently high reactor and/or regenerator temperatures (e.g., above about 300xc2x0 C. to effect the desired chemical phase transformations, this heat treatment step, likewise, may occur within the process reactor vessel itself.
These two production methods A and B, can be carried using the following generalized step-by-step procedures:
It also should be noted that either of these two general methods A and B can include the use of minor amounts (e.g., less than about 20%) of other components such as binders, fluxes, surfactants and gas evolution agents. These compositions can also include other minor components and especially those used to assist in the regeneration of the sorbed, reduced sulfur gas species. For example, such compositions may include compounds containing Ni, Co, Mo, Cn, Tn, Mn, Fe, V, Cu and combinations thereof. These other minor components also can be added to the resulting composition (e.g., by impregnation or spraying particles of such compositions), or they can be employed by their inclusion in the respective starting ingredient formulations.
Thus, in general terms applicant has discovered a process for removing a reduced sulfur gas from a process stream wherein said process comprises contacting a process stream with a reduced sulfur gas sorbing composition comprising, in the same particle, a zinc titanate phase and a metal oxide-aluminate phase in order to remove at least a portion of the reduced sulfur gas from the process stream. The metal oxide-aluminate phase of applicant""s sulfur sorbing compositions have the general formula MO wherein M is preferably a metal selected from the group consisting of magnesium, zinc, nickel and calcium and O is oxygen. For example, the metal oxide-aluminate phase may be zinc oxide-aluminate, calcium oxide-aluminate, magnesium oxide-aluminate, and so on. Indeed, this metal may be selected from a wide variety of divalent and/or trivalent metals.
In some preferred embodiments of this invention, the reduced sulfur gas composition, after sorption of a reduced sulfur gas, is regenerated by contacting it with an oxygen-containing gas (such as air) at an elevated temperature, in order to desorb the reduced sulfur species and thereby regenerate the sulfur sorbing composition for subsequent reduced sulfur gas sorption duty. Preferably, the reduced sulfur gas sorbing composition has a weight ratio of zinc titanate to metal oxide-aluminate phase ranging between about 5:80 and about 95:20.
These sulfur sorbent compositions may be prepared from a zinc titanate ingredient and at least one metal oxide-aluminate having the general formula MAlO, where M is a metal selected from the group consisting of magnesium, zinc, nickel and calcium, Al is aluminum, O is oxygen and where the zinc titanate ingredient and metal oxide-aluminate ingredient are in weight ratios of from about 5:80 to about 95:20. Such compositions may further comprises an inorganic binder in an amount such that it constitutes from about 2.0 to about 15.0 weight percent of the composition. Preferably such inorganic binder(s) will be selected from the group consisting of finely-sized bentonite, kaolinite, forsterite, vermiculite, feldspar, Portland cement, oil shale, calcium sulfate and mixtures thereof.
These compositions may be made by two general processes. The first process generally involves: (a) pre-reacting a zinc containing compound with a titanium-containing compound at sufficient temperature to effect a transformation of zinc and titanium-containing ingredients into a zinc titanate compound; (b) combining the zinc titanate compound prepared in Step (a) with a metal oxide-containing compound and a aluminum-containing compound; (c) forming the zinc titanate/metal oxide aluminate composition created by step (b) into a desired shape such as extrudates, Microspheres, granules, pellets, powder; and (d) heating shaped particles created by step (c) to a temperature greater than about 300xc2x0 C. for a time of greater than one minute. In the alternative the zinc titanate compound may be obtained from commercial sources.
Applicant""s second general process generally involves: (a) combining a zinc-containing compound, a aluminum-containing compound, a titanium-containing compound and a metal oxide-containing compound to form a slurry, past, etc.; (b) forming the resulting zinc titanate/metal oxide-aluminate precursor material into a desired shaped; and (c) heating the resulting shaped material to a temperature greater than about 300xc2x0 C. for a time of greater than one minute.
In some of the more preferred embodiments of this invention, the use of applicant""s compositions may be improved through such measures as (1) constantly recirculating the composition in a fluid bed reactor to effect sorption of the reduced sulfur gas, (2) extracting a portion of partially sorbed particles and subjecting them to a regeneration step and (3) regeneration of the composition by ceasing a gas flow in said process and then subjecting the sorbent composition to a regeneration step.